Method for Preparing Porous Metal Material and Application Thereof

ABSTRACT

A method for preparing a porous metal material comprises: in a vacuum environment, volatilizing one or more volatile alloy elements in an alloy, so as to finally form a porous pure metal or a porous alloy. The process method can be widely applied in the fields such as aeronautics and astronautics, atomic energy, electrochemistry, petrochemical industry, metallurgy, machinery, medicines, environmental protection or construction.

FIELD OF INVENTION

The present invention relates to the technical field of metallic material processing, and more particularly to a method for preparing a new porous metal material and applications thereof.

DESCRIPTION OF RELATED ARTS

The significant feature of the porous metal material is that the porous metal material has a large member of pores therein, and the internal pores of the porous metal material enable the porous metal material to have many excellent characteristics, such as small proportion, large specific surface area, good energy absorption, high heat exchange and dissipation capacity, good sound adsorption, good permeability and good electromagnetic wave absorption. Porous organic polymer materials have a low strength and bad high temperature tolerance and porous ceramic materials are brittle and cannot resist thermal shock. Therefore, the porous metal materials are widely used for separation, filtration, catalysis, electrochemical process, silencers, shock-absorbing, shielding, heat exchange processes in the fields such as aeronautics and astronautics, atomic energy, electrochemistry. petrochemical industry, metallurgy, machinery, medicines, environmental protection or construction, which are made into filters, catalysts and catalyst supports, porous electrode, energy absorbers, mufflers, shock absorber buffer, electromagnetic shielding devices, electromagnetic compatibility devices, heat exchangers and fire-retardant and so on.

The current preparing methods for porous metal materials are as follows: melt foaming method, solid-gas eutectic solidification, powder foaming method, casting method, spray foaming method, sintering metal powder or fiber method, pulp sponge soaked sintered method, electrodeposition method, vapor deposition method and so on. In recent years, dealloying method has become a method for preparing nano-porous materials, wherein the porous materials of nano-porous molybdenum, nano-porous palladium, nano-porous titanium and nano-porous copper can be prepared by selective etching. However, this method belongs to the dealloying corrosion, and when it is used for preparing porous metal materials, the process is generally carried out on internal layers of the metal materials and difficult to prepare a bulk of porous material.

SUMMARY OF THE PRESENT INVENTION

The main object of the present invention is to provide a method for preparing a porous metal material having a three-dimensional through-hole structure, wherein the porous metal material prepared by the process can be widely used in the fields such as separation, filtration, catalysis, noise elimination, battery collector, capacitor, energy absorption and shock reduction, optical thin-film electromagnetic shielding, heat exchange and medical plastic.

The present invention provides a method for preparing a porous metal material, characterized in that: in a vacuum environment, volatilizing one or more volatile alloying elements, so as to finally form a porous pure metal or alloy.

The basic principle of the present invention is utilizing one or more volatile alloying elements (pore-forming element) having a relatively high vapor pressure (at least three orders of magnitude higher than other elements at a same processing temperature) in a specific temperature range, sustaining high vacuum evaporation to volatilize gradually the volatile alloying element so as to finally form a porous pure metal or alloy. Therefore, the raw materials must be an alloy, and it has at least one pore-forming element, wherein the pore-forming element has a high vapor pressure relative to the basic element, wherein the pore-forming element (elements) and the basic element (elements) can form event alloy, solid solution or mixture prepared by powder metallurgy process, wherein the content of the pore-forming element in the alloy may be controlled to 20 at %-80 at % to form different porosity and finally volatilize the pore-forming element at a temperature below the melting point of the alloy and in a vacuum environment.

The method of the present invention, which is used for preparing a porous metal material, characterized in that: in order to save energy and improve efficiency, preferably control the thickness of the alloy to be 0.005 mm˜1000 mm and keep the alloy warm for no less than 0.1 h at a temperature of 200° C.˜1200° C. and in a vacuum environment having a air pressure no more than 500 Pa according to the thickness thereof to volatilize the volatile alloying element gradually, wherein the lower the pressure of the vacuum environment becomes, the better the volatilize effect is. Preferably, the air pressure of the vacuum environment is no more than 10 Pa.

The method of the present invention, which is used for preparing a porous metal material, characterized in that: the process temperature is lower than the melting point of the alloy, wherein the process temperature is no lower than the temperature when the vapor pressure of the volatile alloying element is no less than 0.1 Pa and no higher than 85% of the melting point of the alloy (for example, the melting point of brass is 930° C., so the process temperature is no higher than 790.5° C., otherwise the pores will be closed).

The method of the present invention, which is used for preparing a porous metal material, characterized in that: the preparation process is as follow: employing a commercially available or self-manufactured alloy, wherein the alloy contains at least one or more volatile alloying elements; according to the different melting points of different allies and the vapor pressures of the different volatile alloying elements thereof, volatilizing the volatile alloying element from the alloy by maintaining the alloy at a temperature no more than the melting point thereof and holding continually the alloy in a high vacuum environment, wherein the vacuum degree of the vacuum environment is preferably no more than 10 Pa, so as to finally form a porous pure metal or alloy; wherein the vapor pressure of the volatile alloying element in the alloy has to be more than (in a same processing temperature, at least three orders of magnitude) the basic alloying element in the alloy, and the volatile alloying element in the alloy is capable of forming an even alloy or solid solution. Or after mixing various metal powders by powder metallurgy process and pressing them to form the metal mixture haying a required shape.

The alloy or metal mixture of the present invention, characterized in that: the alloy or metal mixture must be relatively uniform, the uniformity of the excellent commercial alloy can meet the processing demand, and the uniformity of the self-manufactured alloy may need to be improved by stirring or repetitious re-melting when the alloy is smelted and manufactured, or the metal mixture need to be stirred fully before pressing.

The alloy of the present invention may be ferrous alloy, nickel base alloy, titanium alloy, cobalt-base alloy, copper alloy, noble metal alloy, silicon alloy, aluminum or magnesium alloy, the pore-forming element employs the volatile alloying element, preferably manganese, zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, ytterbium, magnesium, calcium, thallium, barium, bismuth, potassium, lead, sulfur, phosphorus, sodium, lithium and the like, which has a high saturation vapor pressure. The alloy may also be gold, platinum, rhodium, palladium or iridium-based precious metal alloy, which selects one or more elements of manganese, zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, ytterbium, magnesium, calcium, thallium, barium, bismuth, potassium, lead, sulfur, phosphorus, sodium, lithium elements as the volatile alloying element thereof.

The pore-forming element must be employed according to the characteristic of the manufactured porous metal alloy, for example, the ferrous alloy, the titanium alloy, the cobalt-base alloy and the silicon alloy should preferably employ magnesium as the pore-forming element thereof, and the copper alloy preferably employs zinc, strontium or cadmium as the pore-forming, element thereof, the aluminum or magnesium alloy preferably selects zinc, cadmium, arsenic, bismuth, potassium, sulfur, phosphorus, sodium, antimony, tellurium, selenium or strontium element as the volatile alloying element thereof.

The raw alloy materials for preparing the porous metal alloy material may be prepared. by smelting, metallurgy and other methods, wherein the raw alloy materials must be polished to remove the surface oxide layers thereof, before they are used. The surface processing method, for example, iron plating or spraying process can be employed to prepare an alloy layer on a surface of a common metal so as to prepare a graded material having a porous surface by the manufacturing method of the present invention. The manufacturing method of the present invention can be employed to make an alloy powder having a grain size larger than 10 um be prepared into a porous alloy powder or sphere.

The porous metal material prepared by the manufacturing method of the present invention has a wide pore size distribution range, which may be up to 0.01 um˜100 um (generally 0.1 um˜20 um), and The porous metal material has an average pore size of 2 um˜5 um and is uniform.

The manufacturing method of the present. invention can be used for separation, filtration, catalysis, electrochemical process, silencers, shock-absorbing, shielding, heat exchange processes in the fields of aeronautics and astronautics, atomic energy, electrochemistry, petrochemical industry, metallurgy, machinery, medicines, environmental protection or construction, which are used for preparing filters, catalysts and catalyst supports, porous electrode, energy absorbers, mufflers, shock absorber buffer, electromagnetic shielding devices, electromagnetic compatibility devices, heat exchangers and fire-retardant and so on.

The present invention has the following advantages:

(1) the mature vacuum heat treatment process is suit to the large-scale production, which is not only used for preparing the large blocks, but also particularly suitable for the preparation of ultra-thin metal foils, powders or spheres, as well as a variety of metal tubes, wherein the ultra-thin metal foils are light-transmitting and breathable and can be used for filtering.

(2) the porous pure metal or alloy prepared by the manufacturing method of the present invention has a three-dimensional through-hole structure porous pure metal or an alloy prepared by the present process has a through hole structure, wherein the porosity of the porous pure metal or alloy is adjustable according to the proportion of the alloy.

(3) the manufacturing method of the present invention with can also be used for preparing porous surface gradient materials or porous powder materials.

(4) the porous pure metal or alloy of the present invention can be used in the fields of battery current collector, separation, filtration, catalysis, silencers, shock-absorbing, shielding, heat exchange and so on.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a three-dimensional porous pure copper material (2 um) according to a first preferred embodiment of the present invention.

FIG. 2 illustrates a three-dimensional porous copper alloy material (5 um) according to a third preferred embodiment of the present invention.

FIG. 3 illustrates a three-dimensional porous nickel alloy material (5 um) according to a third preferred embodiment of the present invention.

FIG. 4 illustrates a three-dimensional porous stainless steel alloy material (10 um) according to a fifth preferred embodiment of the present invention.

FIG. 5 illustrates a three-dimensional porous silicon alloy material (5 um) according to a sixth preferred embodiment of the present invention.

FIG. 6 illustrates a three-dimensional porous pure copper powder (50 um) according to a seventh preferred embodiment of the present invention.

FIG. 7 illustrates a three-dimensional porous pure copper foil (5 um) according to an eighth preferred embodiment of the present invention.

FIG. 8 shows charge-discharge test results.

FIG. 9 illustrates a porous copper wire (20 um) according to a ninth preferred embodiment of the present invention.

FIG. 10 illustrates a porous copper tube (5 um) according to a tenth preferred embodiment of the present invention.

FIG. 11 illustrates a porous copper sheet (10 um) according to a comparative example of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following embodiments of the present invention will be only for illustrating the present invention and not intended to be limiting.

Unless specifically stated, any percentage of the embodiments of the present invention represents an atomic percentage.

Embodiment 1

Employing commercial available 62 brass, which is made into 20×20×1 mm small pieces and the small pieces are suspended in a small vacuum heat treatment furnace used in lab; keeping them warm at 600° C. for 3 hours in a gradual high vacuum environment, wherein the degree of vacuum is controlled within 10 Pa, to prepare three-dimensional porous copper pieces (as shown in FIG. 1), wherein the copper pieces have a pore size of 1 um˜3 um and a porosity of about 20%.

Embodiment 2

Employing commercial available 62 brass, which is made into 20×20×1 mm small pieces and the small pieces are suspended in a small vacuum heat treatment furnace used in lab; keeping them warm at 800° C. for 2 hours in a continual high vacuum environment, wherein the degree of vacuum is controlled within 10 Pa. No three-dimensional porous copper piece is produced. The copper pieces are analyzed by energy spectrum and found that although zinc in brass is completely released, the samples have only a few pores in the surfaces of the samples. The reason is that the over-high processing temperature results in the diffusion and fusion of the pores formed in the surface of the brass, so a suitable processing temperature must be selected to prepare a corresponding porous metal alloy according different alloys.

Embodiment 3

Employing self-manufactured 40 silicon brass (60% zinc, 3% silicon), melting the prepared pure copper, pure zinc and pure silicon in a heat treatment furnace by utilizing a graphite crucible, in consideration of volatilization of zinc, additional 2% zinc content is specially added, pouring and forging to obtain a metal block, and then linear cutting the block into 10×15×1 mm sheets, sanding them to have a thickness of 0.8 mm, and then suspending them in a small vacuum heat treatment furnace used in lab; keeping them warm at 500° C. for 1 hour, wherein the degree of vacuum is controlled within 10 Pa, to prepare three-dimensional porous copper-silicon alloy (as shown in FIG. 2), wherein the copper-silicon alloy has a pore size of 1 um˜8 um and a porosity of about 40%.

Embodiment 4

Employing self-manufactured nickel-manganese alloy (70% manganese content), smelting in a vacuum induction furnace, wherein the raw materials are pure nickel and electrolytic manganese, the protective gas is argon gas, and in consideration of volatilization of manganese, the manganese content is 72%, the remaining ingredient is nickel, the actual measured content of manganese is 69.5%; pouring and linear cutting the ingot into 10×15×1 mm sheets; sanding them to have a thickness of 0.8 mm, and then suspending them in a small vacuum heat treatment furnace used in lab; keeping them warm at 900° C. for 1 hour, wherein the degree of vacuum is controlled within 10 Pa, to prepare three-dimensional porous pure nickel (as shown in FIG. 3), wherein the porous pure nickel sheets have a pore size of 2 um˜10 um and a porosity of about 40%.

Embodiment 5

Employing self-manufactured manganese 316 stainless steel alloy (50% manganese content); smelting the prepared 316 stainless steel and electrolytic manganese in a vacuum induction furnace, and in consideration of volatilization of manganese, the manganese content is 51%, the remaining ingredient is 316 stainless steel, the actual measured content of manganese is 50.5%; pouring and linear cutting the ingot into 10×15×1 mm sheets; sanding them to have a thickness of 0.8 mm, and then suspending them in a small vacuum heat treatment furnace used in lab; keeping them warm at 1000° C. for 1 hour, wherein the degree of vacuum is controlled within 5 Pa, to prepare three-dimensional porous stainless steel (as shown in FIG. 4), wherein the porous stainless steel sheets have a pore size of 2 um˜15 um and a porosity of about 50%.

Embodiment 6

Employing self-manufactured silicon manganese alloy (60% manganese content); smelting the prepared pure silicon and electrolytic manganese in a vacuum induction furnace, and in consideration of volatilization of manganese, the manganese content is 63%, the remaining ingredient is silicon, the actual measured content of manganese is 60.2%; pouring and linear cutting the ingot into 10×5×1 mm sheets, and then suspending them in a small vacuum heat treatment furnace used in lab; keeping them warm at 900° C. for 2 hours in a high vacuum environment, wherein the degree of vacuum is controlled within 10 Pa, to prepare three-dimensional porous silicon (as shown in FIG. 5), wherein the porous silicon sheets have a pore size of 2 um˜10 um and a porosity of about 15%.

Embodiment 7

Employing commercial available 62 brass powders, wherein the brass powder has a size of 100 mesh; placing them in a small vacuum heat treatment furnace used in lab, wherein the degree of vacuum is controlled within 10 Pa, to prepare three-dimensional porous pure copper powders, wherein the porous pure copper powders have a pore size of 2 um˜10 um, as shown in FIG. 6.

Embodiment 8

Employing commercial available 62 brass sheets having a thickness of 20 um; cutting them into 100×100 mm sheets, and then placing them in a small vacuum heat treatment furnace used in lab; keeping them warm at 550° C. for 1 hour in a high vacuum environment, wherein the degree of vacuum is controlled within 10 Pa, to prepare three-dimensional porous pure copper sheets, wherein the porous copper sheets have a pore size of 2 um˜10 um (as shown in FIG. 7), and then utilizing the porous copper sheets as current collectors, and selecting LiCoO₂ as positive electrode material, composite graphite as negative material, wherein the electrolyte employs commercial available electrolyte of 1 mol/L LiPF₆/EC+DMC+EMC (1:1:1 mass ratio), pressing them into button cells in an argon atmosphere glove box, wherein the charge-discharge test results of the button cells are shown in FIG. 8.

Embodiment 9

Employing a commercial available 62 brass wire, placing it in a small vacuum heat treatment furnace used in lab, keeping them warm at 550° C. for 1 hour in a high vacuum environment, wherein the degree of vacuum is controlled within 10 Pa, to prepare three-dimensional porous pure copper wire, as shown in FIG. 9.

Embodiment 10

Employing a commercial available 62 brass tube, which has an external diameter of 2 mm and a wall thickness of 0.1 mm, placing it in a vacuum heat treatment furnace used in lab, keeping them warm at 600° C. for 2 hours in a high vacuum environment, wherein the degree of vacuum is controlled within 10 Pa, to prepare a three-dimensional porous copper tube, as shown in FIG. 10.

Comparative Example

Employing commercial available 62 brass, making it into 20×20×1 mm sheets, and then suspending them in a small vacuum heat treatment furnace used in lab; keeping them warm at 800° C. (temperature being on the high side, being higher than 85% of 930° C.) for 3 hours in a high vacuum environment, wherein the degree of vacuum is controlled within 10 Pa, wherein most of pores in the surfaces of the prepared copper sheets are closed, and there are a few of pores in the surfaces of the prepared copper sheets, as shown in FIG. 11.

The above embodiments are provided to illustrate the technical conception and features so as to enable any person skilled in the art to understand and implement the present invention, and not to limit the scope of the present invention. The equivalents and modifications without departing from the spirit and scope of the present invention should be within the scope of the present invention. 

1. A method for preparing a porous metal material, comprising: volatilizing one or more volatile alloy elements in the alloy in a vacuum environment, so as to finally form a porous pure metal or a porous alloy, wherein the alloy has at least one pore-forming element, wherein the pore-forming element has a higher vapor pressure relative to the basic element of the alloy, and the pore-forming element and the basic element can form the event alloy, solid solution or mixture prepared by powder metallurgy process.
 2. The method, as recited in claim 1, characterized in that: the volatile alloy element of the alloy is volatilized at a temperature less than the melting point of the alloy and in a continual vacuum environment.
 3. The method, as recited in claim 1, characterized in that: the alloy is a commercially available or self-manufactured alloy and the alloy is placed under a temperature no more than the melting point thereof and in a continual vacuum environment to volatilize gradually the volatile alloying element from the alloy, wherein the vacuum degree of the vacuum environment is maintain within 10 Pa, so as to finally form a porous pure metal or alloy, wherein the vapor pressure of the volatile alloying element of the alloy is at least three orders of magnitude higher than the basic alloying element of the alloy.
 4. The method, as recited in claim 1, characterized in that: the atomic percentage of the volatile alloy element in the alloy is control to 20%˜80%.
 5. The method, as recited in claim 1, characterized in that: the alloy is controlled to have a thickness of 0.005 mm˜1000 mm, wherein the alloy is placed under a temperature range 200° C.˜1200° C. and a vacuum degree of no more than 10 Pa, wherein the alloy is kept warm for no less than 0.1 hour according to the thickness thereof to volatilize gradually the volatile alloying element.
 6. The method, as recited in claim 1, characterized in that: the alloy is ferrous alloy, nickel base alloy, titanium alloy, cobalt-base alloy or copper alloy, and at least one of manganese, zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, ytterbium, magnesium, calcium, thallium, barium, bismuth, potassium, lead, sulfur, phosphorus, sodium and lithium is used as the pore-forming element of the alloy; or the alloy is noble metal alloy, and at least one of manganese, zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, ytterbium, magnesium, calcium, thallium, barium, bismuth, potassium, lead, sulfur, phosphorus, sodium and lithium is used as the pore-forming element of the noble metal alloy, wherein the noble metal alloy selects gold, platinum, rhodium, palladium or iridium as the based element of the noble metal alloy; or the alloy is aluminum alloy or magnesium alloy, and at least one of zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, bismuth, potassium, sulfur, phosphorus and sodium is used as the pore-forming element of the alloy; wherein the alloy is prepared by smelting or metallurgy method, and the alloy is polished to remove the surface oxide layer thereof, before it is used, be polished to remove the surface oxide layer thereof.
 7. The method, as recited in claim 1, characterized in that: the process temperature is no lower than the temperature when the vapor pressure of the volatile alloying element is no less than 0.1 Pa, and the process temperature is no higher than 85% of the melting point of the alloy.
 8. A porous metal material prepared by the method as recited in claim 1, characterized in that: the porous metal material has a pore size range 0.1 um˜20 um.
 9. An application of the method as recited in claim 1, characterized in that: the preparing method is adapted for being used in the field of aeronautics and astronautics, atomic energy, electrochemistry, petrochemical industry, metallurgy, machinery, medicines, optical, environmental protection and construction.
 10. An application of the method as recited in claim 1, characterized in that: the porous metal alloy prepared by the method is adapted for being used for separation, filtration, catalysis, shock-absorbing, battery current collector, capacitor, energy absorption and shock reduction, optical thin-film, electromagnetic shielding, heat exchange and medical plastic.
 11. The method, as recited in claim 2, characterized in that: the atomic percentage of the volatile alloy element in the alloy is control to 20%˜80%.
 12. The method, as recited in claim 3, characterized in that: the atomic percentage of the volatile alloy element in the alloy is control to 20%˜80%.
 13. The method, as recited in claim 2, characterized in that: the alloy is controlled to have a thickness of 0.005 mm˜1000 mm, wherein the alloy is placed under a temperature range 200° C.˜1200° C. and a vacuum degree of no more than 10 Pa, wherein the alloy is kept warm for no less than 0.1 hour according to the thickness thereof to volatilize gradually the volatile alloying element.
 14. The method, as recited in claim 3, characterized in that: the alloy is controlled to have a thickness of 0.005 mm˜1000 mm, wherein the alloy is placed under a temperature range 200° C.˜1200° C. and a vacuum degree of no more than 10 Pa, wherein the alloy is kept warm for no less than 0.1 hour according to the thickness thereof to volatilize gradually the volatile alloying element.
 15. The method, as recited in claim 2, characterized in that: the alloy is ferrous alloy, nickel base alloy, titanium alloy, cobalt-base alloy or copper alloy, and at least one of manganese, zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, ytterbium, magnesium, calcium, thallium, barium, bismuth, potassium, lead, sulfur, phosphorus, sodium and lithium is used as the pore-forming element of the alloy; or the alloy is noble metal alloy, and at least one of manganese, zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, ytterbium, magnesium, calcium, thallium, barium, bismuth, potassium, lead, sulfur, phosphorus, sodium and lithium is used as the pore-forming element of the noble metal alloy, wherein the noble metal alloy selects gold, platinum, rhodium, palladium or iridium as the based element of the noble metal alloy; or the alloy is aluminum alloy or magnesium alloy, and at least one of zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, bismuth, potassium, sulfur, phosphorus and sodium is used as the pore-forming element of the alloy; wherein the alloy is prepared by smelting or metallurgy method, and the alloy is polished to remove the surface oxide layer thereof, before it is used, be polished to remove the surface oxide layer thereof.
 16. The method, as recited in claim 3, characterized in that: the alloy is ferrous alloy, nickel base alloy, titanium alloy, cobalt-base alloy or copper alloy, and at least one of manganese, zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, ytterbium, magnesium, calcium, thallium, barium, bismuth, potassium, lead, sulfur, phosphorus, sodium and lithium is used as the pore-forming element of the alloy; or the alloy is noble metal alloy, and at least one of manganese, zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, ytterbium, magnesium, calcium, thallium, barium, bismuth, potassium, lead, sulfur, phosphorus, sodium and lithium is used as the pore-forming element of the noble metal alloy, wherein the noble metal alloy selects gold, platinum, rhodium, palladium or iridium as the based element of the noble metal alloy; or the alloy is aluminum alloy or magnesium alloy, and at least one of zinc, arsenic, cadmium, antimony, tellurium, selenium, strontium, bismuth, potassium, sulfur, phosphorus and sodium is used as the pore-forming element of the alloy; wherein the alloy is prepared by smelting or metallurgy method, and the alloy is polished to remove the surface oxide layer thereof, before it is used, be polished to remove the surface oxide layer thereof.
 17. The method, as recited in claim 2, characterized in that: the process temperature is no lower than the temperature when the vapor pressure of the volatile alloying element is no less than 0.1 Pa, and the process temperature is no higher than 85% of the melting point of the alloy.
 18. The method, as recited in claim 3, characterized in that: the process temperature is no lower than the temperature when the vapor pressure of the volatile alloying element is no less than 0.1 Pa, and the process temperature is no higher than 85% of the melting point of the alloy. 